Optical discs such as CDs and DVDs are sold and rented to consumers. The content of the optical discs may be music, movies, video clips, software or data. The purchase price of CDs and DVDs can be high; this reflects the value of the information encoded on the discs, such as movies or software, rather than the manufacturing cost of these optical discs. Frequently, content providers, such as movie studios or software companies, do not want to sell at a low cost copies of their information that will have a long lifetime in the marketplace. Consumers frequently want to access content information only for a brief period and at low cost. Rentals of CDs and DVDs enable consumers to access content information at a lower cost than if consumers had to purchase the media, but the need to return the physical media is inconvenient. It would be desirable to have limited play/expiring optical media that the user could purchase at a low cost, would address the concerns of the content providers about lifetime of their content in the marketplace, and which would not have the disadvantage of having to be returned, as is the case with videotape movie rentals today. It would also he desirable to manufacture such optical media at low cost and with minimum changes to existing manufacturing processes for optical discs. Finally, in order for the content providers to be willing to provide their content through limited play/expiring optical media, the mechanism that limits playing of the media should not be easily detectable, enabling access to the content beyond the intended period of use.
Herefore, the requirements of low cost limited content lifetime, avoidance of rental returns, resistance to attempts to defeat, and minimum changes to existing manufacturing processes referred to above have not been fully met. What is needed is a solution that simultaneously addresses all of these requirements. One embodiment of the present invention is directed to meeting these requirements, among others.
Several approaches have been proposed to make a limited play (expiring) optical disc based on a layer that changes from a non-interfering (“transparent”) state where it does not interfere with the reliable reading of the information on the optical disc to an interfering (“opaque”) state where the layer interferes with the reading of the data on the optical disc (e.g., see U.S. Pat. No. 5,815,484 assigned to Smith et al. and U.S. Pat. No. 6,011,772 assigned to Rollhauss et al.). The interference may be due to the layer becoming dark, reflective, highly birefringent, pitting, bubbling, shattering, corroding, bending, changing refractive properties or combinations of these, among other possibilities.
Optical discs with such a layer changing from a transparent to an opaque state in response to a stimulus such as exposure to oxygen in the atmosphere, or the light of the reading laser, can be used to manufacture limited-play optical discs (such as DVDs) that become unusable in a predetermined way (such as within a certain period of exposure to environmental oxygen). Such discs can find a variety of commercial applications, such as the viewing of a video by consumers at a moment chosen by the consumer and without the need to return the expired optical disc.
The interfering layer that renders the disc unplayable by inhibiting the reading of the data can be applied via a variety of techniques to the surface of an optical disc. Such an approach, however, has a number of disadvantage. For example, it may be defeated by finding a way to reverse the transition of the layer to an opaque state, such as exposing the disc to a reducing chemical substance that reverses an oxidation reaction, or by entirely removing the layer through chemical means (such as solvents) or mechanical means (such as polishing or grinding). Also, adding an additional layer can complicate manufacturing of the optical discs, for example by requiring additional capital equipment and additional steps in the manufacturing process, and thus can increase the costs and/or decrease the yields for the manufacturing of optical discs.
A protective layer engineered to resist attempts to defeat the disc can be applied on top of the interfering layer, an approach that has been used by at least some of the present inventors. However, this introduces still another step in the manufacturing process, further adding to costs and possibly further reducing manufacturing yields. Furthermore, since the protective layer would still be at the surface of the disc, it could still be removed by chemical means (such as solvents) or mechanical means (such as polishing or grinding), or could be defeated by chemical substances that could diffuse through the protective layer and reach the reactive layer.
As explained above, when manufacturing expiring optical discs, it is desirable to employ a cost effective manufacturing process and to make discs that are not easily defeatable. In addition, it is desirable for the disc to make a rapid transition from the playable to the expired state. Among other benefits, this would reduce the variation of the playing period among optical media players and drives, despite the fact that there is substantial variability in the ability of the players and drives in the market to play discs with a given deterioration in their physical playability characteristics (such as the reflectivity to the light of the reading laser).